Purification of magnesium chloride solutions



United States Patent PURIFICATION OF MAGNESIUM CHLORIDE SOLUTIONSCharles H. Fuchsman, Corpus Christi, Tex., assignor to InternationalMinerals & Chemical Corporation, a corporation of New York No Drawing.Application July 26, 1951, Serial No. 238,787

6 Claims. (Cl. 23-91) This invention relates to processes for thetreatment of liquors to recover magneisum values therefrom, and moreparticularly to processes for the treatment of potassium sulfate rejectliquors or their equivalents to recover magnesium compounds therefrom.

In the process of producing potassium sulfate by base exchange ormetathesis of the magneisum content of langbeinite with the potassiumcontent of potassium chloride, there is produced a mother liquor fromwhich the potassium sulfate crystals have been separated. This motherliquor is usually evaporated and cooled in order to separate potassiumchloride and leonite crystals from the liquor. The liquor from which thecrystalline salts have been removed is generally sent to waste, eventhough it contains magnesium and potassium compounds. It would bedesirable to recover saleable magnesium compounds from these potassiumsulfate reject liquors if a commercially feasible process for recoverywere available.

It is an object of the instant invention to provide a process for therecovery of magnesium compounds from liquors containing chlorides andsulfates of magnesium, potassium and sodium.

It is a further object of the instant invention to provide acommercially feasible process for the recovery of magnesium compoundsfrom potassium sulfate reject liquors or their equivalents.

It is a further object of the instant invention to provide acommercially feasible process for the recovery from potassium sulfatereject liquors or their equivalents of magnesium chloride which may beprocessed to obtain other magnesium chemicals.

It is a further object of the instant invention to produce magnesiumchloride, magnesium oxide, and hydrochloric acid from potassium sulfatereject liquors.

These and other objects of the instant invention will become moreapparent upon a fuller understanding of the instant novel process ashereinafter set forth.

The instant invention involves the treatment of liquors containingchlorides and sulfates of magnesium, potassium, and sodium underconditions controlled as herein described in order to remove thepotassium, sodium, and sulfate from the liquor and to recover a mixturecomprising essentially magnesium chloride.

According to the general method employed in the instant novel processfor the recovery of magnesium compounds from mixtures or liquorscontaining sulfates and chlorides of magnesium, potassium and sodium,the liquor, such as potassium sulfate reject liquor, is evaporated to amagnesium concentration of between about 8.0% and about 8.9% by weight.In this range of magnesium con centration, langbeinite precipitates fromthe evaporated mixture and is separated therefrom. To the hot liquorfrom which the solid material has been separated is added a calciumchloride solution under conditions as herein described to precipitateresidual amounts of sulfate in the mixture as anhydrite. Anhydrite isseparated from the liquor. The liquor is then evaporated to a point atwhich substantially all of the halite and all of the sylvite icecombined as carnallite crystallize from the liquor, which is at thispoint nearly saturated with respect to bischofite and has a magnesiumchloride content of between about 36% and about 40% by weight. Solidmaterial is separated from the resulting bischofite-containing liquor.The bischofite liquor is then evaporated at a temperature between about170 C. and about 190 C. to between about 50% and 53% by weight, withconcentrations of 52% to 53% preferred, of magnesium chloride. Thisliquor may be processed in order to obtain flaked magnesium chloridewhich is dried and decomposed by heating to produce magnesium oxide andhydrochloric acid.

The liquors which are employed as feed materials in practicing theinstant process contain chlorides and sulfates of magnesium, potassium,and sodium. In one embodiment of the instant novel process, potassiumsulfate reject liquors such as those produced in base exchange processesas above described and claimed in U. S. Patent No. 2,295,297, issued toButt et al., and pending U. 8. application Serial No. 166,715, filedJune 7, 1950, and the like, are used. A typical potassium sulfate rejectliquor suitable as feed material for the practice of the instant novelprocess contains between about 2.9% and about 3.2% potassium, betweenabout 5.5% and about 6.5% magnesium, between 17% and 18% chlorine,between 5% and 7% sulfate, and small amounts of sodium.

By the term anhydrite as used in the description and claims is meant thesynthetic anhydrous calcium sulfate, which is also known as solubleanhydrite or B-anhydrite. This term does not refer to the naturallyoccurring anhydrite or 'y-anhydrite.

In this embodiment of the instant process, potassium sulfate rejectliquor, having substantially the above analysis, is evaporated at atemperature of between about 100 C. and about 117 C. at atmosphericpressure to. a magnesium concentration of between about 8% and about8.9% by weight. The formation of langbeinite crystals during theevaporation is accompanied by a reduction of the sulfate concentrationin the liquor. The langbeinite crystallizes from the magnesium chlorideliquor which is then decanted or thickened, for example, in theconventional type thickener, which effects the separation of liquidsubstantially free of solids from a mixture containing solids andliquid. The underflow from the thickener, which contains the solid phasesalts, that is langbeinite, sylvite, and halite, can be leached toobtain schoenite and can be recycled to the conventional base exchangeprocess. About of the sulfate ions and about 25% of the potassium valuesare removed in the underfiow.

To the hot magnesium chloride-containing overflow liquor from thethickener is added an aqueous solution containing sufficient calciumchlorideto remove the residual amount of sulfate ion remaining in theliquor. The overflow from the langbeinite thickener is heated to atemperature of between about C. and about 115 C. and preferably to atemperature of about C. prior to the addition of the calcium chloridesolution. When calcium chloride is added to a hot langbeinite liquor,either polyhalite or anhydrite, or both, may form. Because polyhalitedoes not settle at a reasonable rate in the thickener and because iteffects the removal of potassium sulfate, in practicing the instantnovel process conditions are controlled so that anhydrite rather thanpolyhalite is formed. In the liquor employed in the instant process,increasing the temperature of the mixture and/or in creasing thedilution of the mixture minimizes the formation of polyhalite.

In a preferred embodiment of the instant invention prior to the additionof the calcium chloride solution, the magnesium chloride-containingoverflow from the thickener is heated to a temperature of between about100 C.

and about 110 C. and diluted to a magnesium concentration of betweenabout 6.7% and about 8%, preferably between about 7.2% and about 7.5%.In this step, temperature and concentration are closely controlled toeffect anhydrite crystallization. The anhydrite which forms is separatedby means of a thickener.

The overflow liquor from the anhydrite thickener is evaporated to apoint near saturation with respect to bischofite by heating the liquorunder vacuum or at atmospheric boiling temperatures. A temperature inthe range between about 100 C. and about 115 C. at atmospheric pressureis preferably employed, although temperatures between about 60 C. andabout 125 C., either under vacuum or at atmospheric pressure, may beemployed. Carnallite and halite crystallize from the liquor and areseparated from the magnesium chloride-containing liquor by means of athickener. The underflow slurry containing carnallite and halite can beleached with water to recover the magnesium chloride content, and theleach solution is recycled to the langbeinite evaporation step.

The magnesium chloride-containing overflow from the thickener isevaporated to a magnesium concentration of between about 12.5% and about13.2% by weight at a temperature of between about 170 C. and about 190C., 12.5% magnesium concentration corresponding to approximately 50%magnesium chloride concentration and 13.2% magnesium concentrationcorresponding to approximately 53% magnesium chloride concentration.This evaporation step is carried out at a temperature between about 170C. and about 190 C. because if the evaporator product is to be flaked,the amount of water which is driven off must be carefully controlled.When temperatures below about 170 C. are employed, an insufficientamount of water is removed and the evaporator product will not readilysolidify in the subsequent flaking step. When temperatures above about190 C. are employed, usually too much water is removed and the mixturecrystallizes in the evaporator. The evaporated mixture is flaked bypouring the mixture onto a cold surface and flaking the solidified massfrom the surface. The flaked magnesium chloride is decomposed by heatingat a temperature between about 400 C. and about 700 C. in a kiln, andmagnesium oxide and hydrochloric acid are recovered therefrom.

As a specific example of the process, the following serves merely asillustration and it is not intended that the scope of the invention belimited thereto.

EXAMPLE Potassium sulfate reject liquor, having an analysis ofapproximately 3.0% potassium, 6.5% magnesium, 0.7% sodium, 18.3%chlorine, 6.0% sulfate, and about 65.5% water, was evaporated at atemperature between about 105 C. and 110 C. to the extent of about 26%by weight of the reject liquor. Solid langbeinite crystallized from theliquor and was separated from the liquor in a thickener. The underflowfrom the thickener consisted of about 59.5% solid phase and 40.5% liquidphase. About 80% of the sulfate and about 52% of the potassium valueswere removed in the underflow.

The composition of the solid phase in the underflow was calculated asfollows (per 100 parts of thickener underflow):

K Mg Na G1 S04 .4 7.2% by weight. An aqueous solution containing about40% of calcium chloride was added to the hot solution. The anhydritewhich formed was separated in a thickener as the underflow. The liquoroverflowing from the thickener was evaporated at a temperature ofbetween about C. and about C. to a magnesium chloride concentration ofabout 40%. The carnallite and halite present in the evaporated productwere separated as the underflow in a thickener maintained at atemperature of about 105 C. The composition of the mixtures dischargedfrom the thickener as overflow and underflow are shown in Table 1.

The thickener underflow contained about 39% carnallite, 6% halite, and55% liquor. The magnesium chloride-containing overflow liquor was heatedat a temperature between about C. and about C. to a magnesium chlorideconcentration of about 53%, and this concentrate was flaked and heatedin a kiln at a temperature of between about 650 C. and about 700 C. toproduce magnesium oxide and hydrochloric acid. The kiln productcontained about 87% magnesium oxide. The magnesium recovery was about70%.

Having thus fully described and illustrated the character of theinvention, what is desired to be secured and claimed by Letters Patentis:

1. A process for the production of purified magnesium chloride solutionwhich comprises evaporating a potassium sulfate reject liquor whichinitially contains magnesium in a concentration up to about 6.5 byweight to a magnesium concentration between 8 and about 8.9% by weight,separating solid material from the evaporated mixture, diluting theresulting liquor with water to a magnesium concentration of betweenabout 6.7 and 8% by weight, heating the diluted mixture to a temperaturebetween about 100 C. and about 115 C., adding a suflicient amount ofcalcium chloride to the hot liquor to precipitate residual sulfate ionsfrom the solution as anhydrite, separating solid anhydrite material fromthe resulting mixture, evaporating the liquor from which the solidanhydrite material has been separated to magnesium chlorideconcentration of between about 36% and about 40% by weight, andseparating solid material from the resulting magnesiumchloride-containing liquor.

2. A process for the production of purified magnesium chloride solutionwhich comprises evaporating a potassium sulfate reject liquor whichinitially contains magnesium in a concentration up to about 6.5% byweight at a temperature below about 117 C. to a magnesium concentrationof between 8% and about 8.9% by weight, separating solid material fromthe concentrated mixture, diluting the resulting liquor with water to amagnesium concentration of between about 6.7% and 8% by weight, heatingthe diluted mixture to a temperature between about 100 C. and about 115C., adding a suflicient amount of calcium chloride to the hot liquor toprecipitate residual sulfate ions from the solution as anhydrite,separating solid anhydrite material from the resulting mixture,evaporating the liquor from which the solid anhy drite material has beenseparated to a magnesium chloride concentration of between about 36% andabout 40% by weight, and separating solid material from the resultingmagnesium chloride-containing liquor.

3. A process for the production of purified magnesium chloride solutionwhich comprises evaporating a potassium sulfate reject liquor whichinitially contains magnesium in a concentration up to about 6.5% byweight at a temperature between about 100 C. and about 117 C. to amagnesium concentration of between 8% and about 8.9% by weight,separating solid material from the resulting mixture, diluting theresulting liquor with water to a magnesium concentration to betweenabout 6.7 and 8% by weight, heating the diluted mixture to a temperaturebetween about 100 C. and about 115 V., adding sufficient calciumchloride to the hot liquor to precipitate residual sulfate ions in theliquor as anhydrite, separating solid anhydrite material from theresulting mixture, evaporating the resulting solution at a temperaturebetween about 60 C. and about 125 C. to a magnesium chlorideconcentration of between about 36% and about 40% by weight andseparating solid material from the resulting magnesiumchloride-containing liquor.

4. A process for the production of a purified magnesium chloridesolution which comprises evaporating a potassium sulfate reject liquorwhich initially contains magnesium in a concentration up to about 6.5%by weight at a temperature of between 100 C. and about 117 C. to amagnesium concentration of between about 8.0% and about 8.9% by weight,separating solid material from the evaporated mixture, diluting theresulting liquor with water to a magnesium concentration of betweenabout 7.2% and about 7.5% by weight, heating the diluted mixture to atemperature between about 100 C. and about 115 C., adding suificientamount of an aqueous solution of calcium chloride to the hot liquor toprecipitate residual sulfate ions from the liquor as anhydrite,separating solid anhydrite material from the resulting mixture,evaporating the liquor from which anhydrite has been separated at atemperature between about 100 C. and about 115 C. to a magnesiumchloride concentration of between about 36% and about 40% by weight,separating solid material therefrom, heating the resulting magnesiumchloride solution at a temperature between about 170 C. and about 190 C.to obtain a concentrated magnesium chloride solution.

5. A process for the production of purified magnesium chloride solutionwhich comprises evaporating the potassium sulfate reject liquor whichinitially contains magnesium in a concentration up to about 6.5 byweight to a temperature between about 100 C. and about 117 C. to amagnesium concentration of between about 8.3% and about 8.9% by weight,separating solid material from the evaporated mixture, diluting theresulting liquor with water to a magnesium concentration to betweenabout 7.2% and about 7.5% by weight, heating the diluted mixture to atemperature between about C. and about C., adding sufiicient aqueouscalcium chloride solution to the heated liquor in order to precipitateresidual sulfate ions from the liquor as anhydrite, and separating solidanhydrite from the liquor.

6. A process for the production of a purified magnesium chloridesolution which comprises evaporating a potassium sulfate reject liquorwhich initially contains magnesium in a concentration up to about 6.5%by weight at a temperature between about 100 C. and about 117 C. to amagnesium concentration of between about 8.3% and about 8.9% by weight,separating solid material from the evaporated mixture, diluting theresulting liquor with water to a magnesium concentration of betweenabout 7.2% and about 7.5% by weight, heating the diluted mixture to atemperature between about 100 C. and about 115 C., adding sufiicientamount of an aqueous solution of calcium chloride to the heated liquorin order to precipitate residual sulfate ions from the liquor asanhydrite, and separating anhydrite from the resulting liquor,evaporating the liquor from which anhydrite has been separated at atemperature between about 100 C. and about 115 C. to a magnesiumchloride concentration of between about 36% and about 40% by weight,separating solid material therefrom, heating the resulting magnesiumchloride-containing solution at a temperature of between about C. andabout C. to obtain a concentrated magnesium chloride solution.

References Cited in the file of this patent UNITED STATES PATENTS1,871,411 Heath Aug. 9, 1932 2,413,292 Christensen Dec. 31, 19462,437,182 Barr et al. Mar. 2, 1948 2,473,534 Lloyd June 21, 19492,479,001 Burke et al Aug. 16, 1949 2,687,339 Dancy et al Aug. 24, 1954FOREIGN PATENTS 270,620 Germany Feb. 23, 1914

1. A PROCESS FOR THE PRODUCTION PURIFIED MAGNESIUM CHLORIDE SOLUTION WHICH COMPRISES EVAPORATING A POTASSIUM SULFATE REJECT LIQUOR WHICH INTIALLY CONTAINS MAGNESIUM IN A CONCENTRATION UP TO ABOUT 6.5% BY WEIGHT TO A MAGNESIUM CONCENTRATION BETWEEN 8 AND ABOUT 8.9% BY WEIGHT, SEPARATING SOLID MATERIAL FROM THE EVAPORTED MIXTURE, DILUTING THE RESULTING LIQUOR WITH WATER TO A MAGNESIUM CONCENTRATION OF BETWEEN ABOUT 6.7 AND 8% BY WEIGHT, HEATING THE DILUTED MIXTURE TO A TEMPERATURE BETWEEN ABOUT 100* C. AND ABOUT 115* C., ADDING A SUFFICIENT AMOUNT OF CALCIUM CHLORIDE TO THE HOT LIQUOR TO PRECIPITATE RESIDUAL SULFATE IONS FROM THE SOLUTION AS ANHYDRITE, SEPARATING SOLID ANHYDRITE MATERIAL FROM THE RESULTING MIXTURE, EVAPORATING THE LIQUOR FROM WHICH THE SOLID ANHYDRITE MATERIAL HAS BEEN SEPARATED TO MAGNESIUM CHLORIDE CONCENTRATION OF BETWEEN ABOUT 36% AND ABOUT 40% BY WEIGHT, AND SEPARATING SOLID MATERIAL FROM THE RESULTING MAGNESIUM CHLORIDE-CONTAINING LIQUOR. 